The long-term goal of this project remains the purification and characterization of the gamma-aminobutyric acid (GABA) postsynaptic receptor. Recent progress leads us to believe that the simultaneous purification of several GABA receptive sites is possible. Comparison of the pharmacology, Na ion dependence and thermodynamic constants of a sodium dependent (plus Na ion) GABA binding site from synaptic plasma membranes (SPM) with the GABA uptake process in hippocampal synaptosomes leads to the conclusion that these reflect identical systems. Characterization of sodium independent (minus Na ion) GABA binding leads us to conclude that it reflects the binding of GABA to the postsynaptic receptor. Comparison of the two binding sites after purification could produce far reaching implications vis-a-vis structure-function relationships of synaptic and extra-synaptic binding proteins.